This project will evaluate three issues in the clinical development of antagonists of HIV co-receptors using the hu-PBL-SCID animal model for primary HIV-1 infection. The issues are: (1)- How effective are AOP-RANTES and related compounds at preventing infection with different macrophage- tropic (M-tropic) primary isolates, particularly isolates known to differ in affinity for CCR5 binding? What concentrations of chemokine receptor antagonists must be achieved to prevent HIV-1 infection of hu-PBL-SCID mice, and how long must antagonists be administered to prevent infection? Is there a window of time after virus exposure when administration of chemokine receptor antagonists is effective, and how long is that time period? (2)- How effective are AOP-RANTES and related compounds at blocking continued spread of virus in established infection? The efficiency of chemokine receptor antagonists in treating established infection in the hu-PBL-SCID model will be examined, and the target sites of infection will be controlled by altering the cellular composition of the human PBL grant. Frequent monitoring of HIV-1 by plasma viral RNA levels will give a sensitive readout of antiviral effects. (3)- How rapidly do co-receptor switch variants arise during treatment with a single or multiple co-receptor antagonists? We have worked with HIV-1 clones 242 (M-tropic, CCR5 entry) and 241 (dual-tropic, CCR5 + CXCR4 entry) which differ by only a single amino acid in V3 of gp120. We will determine if and how rapidly 242 mutates under selective pressure from AOP-RANTES treatment, and compare that to combined treatment with antagonists for multiple co-receptors. Additional isolates with multiple co-receptor treatment with antagonists for multiple co-receptors. Additional isolates with multiple co-receptor usage (e.g., isolate 89.6) will also be tested in these experiments to determine if usage of alternative co-receptors such as CCR2b or CCR3 is as efficient as use of the primary co-receptors CCR5 and CXCR4. Co-receptors usage variants arising after treatment of primary patient isolates with the most effective combination of antagonists will be isolated (if they occur) and characterized.